The overall objective of these investigations is to study the effects of prolonged administration of pure heroin on the type, extent, persistence and reversibility of the CNS ultrastructural changes, and alterations of chromosomes in the leucocytes, bone marrow and meiotic cells of both males and females in Rhesus (Macaca Mulatta) monkeys. Four basic experiments have been set up: (A) A six-month study of the effects of heroin on pregnant animals and their fetuses for evaluation of heroin's possible effects both on mother and offspring. (B) A twelve-month study of pregnant animals and their fetuses, that will consist of an initial six-month heroin treatment and a six-month "recovery" period, after which the mothers and newborns will be reexamined. These experiments will furnish information on short-term effects and reversibility of the observed findings. (C) An 18-month study on both males and females. (D) A 24-month study on both males and females, that will consist of an initial 24-month heroin exposure and a six-month "recovery" period. These last two experiments will furnish information on the prolonged effects and reversibility of the observed findings.